1. Field of the Invention
The present invention generally relates to an orthogonal frequency division multiplexing (OFDM) receiving apparatus for use in a base station or a terminal station of a radio communication system that performs OFDM radio transmission, or to an OFDM receiving apparatus for use in a receiving station of an OFDM broadcasting system. More particularly, the invention relates to an OFDM diversity receiving apparatus that implements high-quality information transmission even under a poor radio propagation environment.
2. Description of the Background
Terrestrial television broadcasting currently use analog signals. There are plans, however, in about the year 2000, for analog signals to be replaced by digital signals in terrestrial television broadcasting. In terrestrial television broadcasting, the area covered by a single transmission station is very large. A serious problem thus encountered is that multiple reflection wave transmission (multipath) occurs in propagating radio waves, thus degrading image quality. This is referred to as “ghost interference”. The scale of the multipath in terrestrial television broadcasting is much greater than that occurring in radio communication system, such as mobile cellular telephones. Even using an adaptive equalizer, which should be effective as a countermeasure against multipath, the multipath in terrestrial television broadcasting can no longer be handled.
In view of the above background, the OFDM transmission method, which is, in principle, resistant even under a poor multipath transmission environment and achieves high-quality information transmission, is to be employed as a digital transmission method for terrestrial television broadcasting. An OFDM transmission signal is provided with a guard interval in which part of a transmission waveform (symbol) is copied. The guard interval accommodates multipath propagation which is shorter than the length of the guard interval, thereby preventing fatal degradation of the image quality.
There are high expectations that the above-described multipath-resistant transmission characteristics of the OFDM transmission method will be used not only in terrestrial television broadcasting, but also in wide-area radio communication systems, such as multimedia communications, which are to be widely performed in public networks. Accordingly, a technical study of the practical application of the above OFDM transmission method is actively in progress.
Additionally, according to the OFDM transmission method, by virtue of the multipath-resistant transmission characteristics, a single frequency network (SFN) in which the same information is simultaneously transmitted with the same frequency can be constructed. This makes it possible to prevent wasteful use of a frequency, and more specifically, to prevent the transmitting frequency from being changed according to the area, which is conventionally required. In terms of compressing a frequency band for use, the OFDM transmission method is very effective.
However, even though the OFDM transmission method exhibits excellent multipath-resistant transmission characteristics, the receiving characteristics cannot be perfectly maintained because of severe frequency selective fading caused by multipath propagation, or Doppler shift and time fading occurring while a receiving end is moving.
High-definition image transmission is strongly expected to be achieved in digital terrestrial television broadcasting and the next-generation multimedia communications. Particularly in this type of transmission, there is a demand for a radio transmission technique which is much more stable and which exhibits higher quality transmission than that being used in current mobile cellular telephones, in which sound communications rather than image communications are the mainstream. Accordingly, the advent of a receiving method and a receiving apparatus that exhibit higher-quality receiving characteristics is greatly needed.
High-definition images having a large amount of information are required to be transmitted in a narrow radio band in order to effectively and efficiently use the frequency band. It is thus necessary to consider the use of a high-efficiency modulation method, such as a multilevel quadrature amplitude modulation (QAM), under a mobile radio propagation environment. However, a high-efficiency modulation method represented by QAM has poor noise-resistant characteristics and interference-resistant characteristics, and is also vulnerable to distortions.
In a receiving apparatus positioned far from a transmission station or a base station, or in a receiving apparatus receiving signals while it is moving, the received signals have a low signal-to-noise (S/N) ratio or have a propagation distortion. This easily causes degradation of receiving characteristics and consequent, failure to transmit information with a satisfactory quality. This problem becomes serious particularly for terrestrial television broadcasting that covers a very wide area. Accordingly, an effective improvement in the transmission method is required.
A typical measure being taken for improving the receiving characteristics under a severe multiple radio propagation environment or under a poor mobile receiving environment is diversity reception. As a diversity reception method, antenna selection diversity reception is primarily used in which digital signal sequences are demodulated by selecting from a plurality of receiving antennas the receiving antenna having the maximum radio frequency (RF) signal power.
In the OFDM transmission method, however, a received signal is frequency-converted into a baseband signal, which is further converted into frequency spectra in units of symbols. A digital signal sequence is then demodulated from each line spectrum which forms a frequency spectrum. Thus, the antenna selection diversity reception that changes antennas by RF signals is not very effective on the OFDM transmission method. Particularly upon generation of severe frequency selective fading, the receiving characteristics are completely different according to the line spectrum which forms a frequency spectrum. Thus, there is an increasing demand for selection diversity that obtains optimal receiving characteristics for each line spectrum.
As discussed above, in a radio communication/broadcasting system for transmitting digital signal sequences by using the OFDM method, it is necessary to transmit high-quality and high-definition information in all the places within a wide area covered by this system while effectively utilizing the multipath-resistant transmission characteristics of the OFDM transmission method. To satisfy the above requirements, it is further necessary to inhibit the degradation of the receiving characteristics under a severe multiple reflection wave propagation (multipath) environment and to suppress the degradation of the receiving characteristics while a receiving apparatus is moving.
In particular, in future multimedia communications and digital terrestrial broadcasting, in which image transmission is to become the mainstream, the application of a high-efficiency modulation method, such as multilevel QAM, will be essential. Thus, there is an increasing demand for an improvement in the receiving characteristics in the OFDM transmission method, in particular, for the development of an OFDM diversity receiving apparatus that exhibits good receiving characteristics even in response to frequency selective fading.